CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2015	$ 524,938	$ 20,781	$ 108,747	$ 58,433	$ 336,977
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	6,262	266	5,996
Repurchase of common stock	(383)	(17)	(366)
Increase resulting from stock-based compensation expense	37		37
Other comprehensive income	19,954			19,954
Net income	20,782				20,782
Dividends paid to public stockholders	(3,394)				(3,394)
Dividends paid to affiliate	(4,473)				(4,473)
Balance at Jun. 30, 2016	563,723	21,030	114,414	78,387	349,892
Balance at Dec. 31, 2016	553,342	21,223	119,054	46,081	366,984
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	4,129	168	3,961
Repurchase of common stock	(1,758)	(64)	(1,694)
Increase resulting from stock-based compensation expense	30		30
Other comprehensive income	13,299			13,299
Net income	12,308				12,308
Dividends paid to public stockholders	(3,924)				(3,924)
Dividends paid to affiliate	(4,944)				(4,944)
Balance at Jun. 30, 2017	$ 572,482	$ 21,327	$ 121,351	$ 59,380	$ 370,424